The invention is in the field of tools and methods for use in connection with maintained environments found in vapor phase deposition (VPD) processes, such as but not limited to vacuum environments maintained in molecular beam epitaxy (MBE) machines. In particular, the invention is directed to tools and methods for reloading source materials and performing maintenance in VPD environments, such as maintained in an MBE machine, without the necessity of breaking the vacuum of the environment to perform the reload or maintenance operation.
MBE is a technology that was developed in the early 1970""s for the purpose of growing high-purity crystals, particularly epitaxial layers of compound semiconductors. Numerous types of crystal materials may be grown in MBE machines, but the most widely-used application today is III-V compound semiconductors (so called because the two elements used in forming the semiconductor are found in Groups IIIB and VB, respectively, of the periodic table of elements). Gallium Arsenide (GaAs) and other III-V compound semiconductor materials are widely used in optoelectronic components found in cellular telephones, lasers, microwave equipment, and other electro-optical applications.
In the MBE process, the elements that the semiconductors are made from are deposited onto a heated crystalline substrate wafer in the form of xe2x80x9cmolecular beamsxe2x80x9d to form thin epitaxial layers. The molecular beams are formed from thermally evaporated elemental sources. To obtain the necessarily high purity in the epitaxial layers, the material sources must be extremely pure and the entire crystalline growth process must take place in an ultra-high vacuum environment. Also, in order to finely control the deposition of material, the flow of the molecular beams must be precisely controlled. This is generally accomplished using shutters that can open and close in a fraction of a second. Fast shuttering and slow effluence rates makes possible the transition of one material to another at levels which only partially complete an atomic layer. The abrupt epitaxial transitions which can thus be achieved with MBE can be alternated to achieve superlattice structures, wherein some anomalous and highly desirable electrical, optical and magnetic properties may appear.
Most commercial and research MBE machines include at least two major components: a growth chamber and a load chamber. The load chamber is used to bring substrate wafers into and out of the machine while maintaining the vacuum integrity of the growth chambers. The load chamber is also used for the preparation, manipulation, and storage of substrate wafers. The growth chamber is where the MBE process is performed upon the substrate wafers. Effusion cells containing source materials are generally attached to and extend outward from the growth chamber. For certain source materials with very high melting points, electron beam cells may replace effusion cells.
One of the initial problems that was overcome in the design of MBE machines is the transport, storage, and manipulation of substrate wafers. A considerable length of time is required to purify and reestablish a vacuum within the growth chamber once the chamber is opened to the ambient atmosphere. Far less time is required to reestablish a vacuum in the load chamber section once it is opened, provided that the growth chamber remains closed. This is achieved by the use of an air lock between the growth chamber and load chamber. MBE machines have thus been designed so that wafers can be loaded and unloaded without the loss of vacuum in the growth chamber. Wafers are first loaded into the load chamber with the air lock between the load chamber and growth chamber closed. Once the load chamber again reaches a vacuum state, the air lock is opened so that wafers can be moved into the growth chamber. A transport system may be used to move the wafer or wafers through the load chamber. One example of such a system is a chain-driven cart that travels the length of the load chamber. Once the cart reaches the end of the load chamber nearest the growth chamber entrance, a magnetically coupled transfer arm can be used to carry the wafer or wafers lying in the cart from the load chamber into the growth chamber. The arm may be magnetically manipulated by a user situated near the MBE machine. Generally, the arm travels just into the load chamber when retracted, but may be extended into the growth chamber when pushed inward.
The load chamber may also include a vertically oriented manipulation arm, commonly referred to as a xe2x80x9cwobble stick,xe2x80x9d which is generally used to move wafers to and from the load chamber transport means and the magnetically coupled arm. Once the wobble stick has transferred the wafer from the transport means to the magnetically coupled arm, the air lock between the load chamber and growth chamber is opened, and the magnetically coupled arm is extended into the growth chamber to load the wafer into the wafer holder.
The wafer holder in the growth chamber must be heated to maintain the proper temperature at the crystalline substrate wafer. The exact temperature required will depend upon the materials being used. In some systems the wafers are loaded into the growth chamber in an orientation that faces away from the material sources initially, so the wafer holder must flip around to face the wafer toward the material sources before material deposition may begin. The substrate wafer holder and other components that are to be heated must be made of materials that do not decompose or outgas impurities even when heated to high temperatures; such materials as Tantalum (Ta), Molybdenum (Mo), and pyrolytic boron nitride (PBN) have been used for these applications.
MBE machines may be either solid-source or gas-source. Gas-source machines have the advantage of easier reloading of the source material, since all that is required is the replacement of a pressurized bottle holding the gas source material. For a variety of reasons, including output quality and suitability, for some applications solid-source machines are preferred over gas-source machines. There are, therefore, certain MBE applications for which only solid-source materials are used. The solid source material is sublimated within the effusion cell by applying heat. The flux travels out of the effusion cell into the growth chamber as the shutter to the effusion cell opens. In gas-source MBE machines, the sublimation step is of course unnecessary, since a pressure regulator forces the flow of material which is already in the gaseous state to the substrate.
In solid-source MBE machines, the material sources are generally held in PBN crucibles contained within the effusion cells. Each effusion cell may be heated independently to reach the desired flux of the particular material located in that cell. Small changes in flux can significantly affect the epitaxial layer deposition process for some materials, and thus highly accurate thermostats must be used on the effusion cell heaters. Also, the control shutters that open and close the flow of flux from the effusion cells may be computer controlled to allow the cells to be opened and closed quickly and precisely. It should be noted that most MBE machines may accommodate a number of different effusion cells attached to the growth chamber housing, such that numerous types of source materials may be evaporated simultaneously. Since the properties of each material differ, the specific design of each effusion cell must be different in order to properly handle the particular material in question.
Generally, commercial effusion cells may only be heated to a temperature of approximately 1300xc2x0 C. before the PBN crucibles begin to disintegrate. For this reason, some materials cannot be placed in effusion cells because the temperature required to vaporize such materials is too great. Iron (Fe) is an example of one such material. When such materials are to be used, electron beam cells replace the effusion cells. Electron beam cells vaporize source material by exposing the source material to highly energized electrons. In one configuration, a rod of the source material is placed parallel to a filament. A large current is then run through the filament until, due to the filament""s high resistivity, it is heated to a very high temperature. A potential difference is then applied between the filament and rod such that electrons are drawn from the filament toward the rod. These highly energized electrons then strike the surface of the source material, causing it to vaporize.
One of the most significant problems still faced by those using solid-source MBE machines is the long delay caused by the reloading of source materials into the effusion cells and electron beam cells. Typically, the growth chamber of the MBE machine must be xe2x80x9cventedxe2x80x9d (brought up to atmospheric pressure and opened) in order for the cells to be reloaded. Once the source material is used up, the machine is then opened, and each cell is removed so that it may be carefully reloaded by hand. In the case of effusion cells, the delicate PBN crucible within the cell must be refilled with source material. The growth chamber must again be brought to ultra-high vacuum and the environment purified in order to proceed with the MBE process after reloading of source material. Because of the extreme levels of purity and vacuum necessary for the operation of MBE machines, this process requires a substantial amount of time. It is believed that an approximate average for most solid-source commercial production MBE machines building GaAs crystals is a source material reloading downtime of approximately one month for each three months of operation. The cost of this reloading downtime includes not only the cost of labor and materials to perform the repurification of the growth chamber, but also the opportunity cost of missing one month of production, or the cost of purchasing a duplicate machine to continue producing materials while the other machine is undergoing the repurification procedure.
Several solutions to the source material reloading problem have been suggested in the prior art. One solution is to use gaseous source materials rather than solid materials. As already explained, gaseous source materials are not used for some applications. Another partial solution is to simply increase the size of the effusion cell so that it may hold a larger source sample. The principal MBE machine manufacturers have continued to increase sample size with each new generation of MBE machine, but it is believed that the ability to continue increasing sample size has approached its practical limit. For some materials, such as Ga, the size of the sample will affect the ability to maintain accurate temperature control, and thus will effect the quality of the molecular beam. As a result of this problem, the sample size cannot be increased indefinitely without affecting the operation of the machine. Also, as sample sizes increase, powered equipment is necessary to move and load the samples, greatly increasing the cost and complexity of MBE machines.
Another solution that has been attempted is to reload the cells from the back (that is, the end extending outward from the growth chamber) without venting the growth chamber. If an air lock and means to translate the cell is placed between the effusion cell and the chamber, such that the cell can be backed away and sealed from the growth chamber, then the cell can be opened and the material can be placed into the effusion cell by hand without losing the vacuum on the growth chamber. This approach does, however, have several disadvantages. First, since each cell contains a different type of material, the hardware necessary to perform reloading of each cell would necessarily be specialized to that particular cell. A machine with twelve different effusion cells, for example, may require twelve different hardware systems to perform this xe2x80x9cback loadingxe2x80x9d operation, which would greatly increase the cost, complexity, and reliability of the machine and reloading procedure. Second, this approach would require that the cell be removed each time the material is reloaded. As a result of exposing the entire cell to the atmosphere, a lengthy purification process is still required before normal operation of the MBE machine can resume. Finally, this approach would only allow access to the cell during the source material reloading process, and would not allow simultaneous maintenance and cleaning of the growth chamber, or any other maintenance operations on the front side of the air lock separating the cell from the growth chamber. Therefore, a means to reload source material in maintained environments, including the ultra-high vacuum environments of solid-source MBE machines, where the growth chamber need not be vented and cleaning and maintenance within the growth chamber may be performed, is desired.
The invention comprises an apparatus and method to perform activities in a vapor phase deposition environment, like a vacuum environment found in a solid-source MBE machine, by the use of a tool inserted through a port on the load chamber and extending into the growth chamber; or attached directly to the growth chamber. Such tools can thus be used to reload every effusion or electron beam cell in an MBE machine. The tools are constructed of a material, such as Tantalum or Molybdenum, that can withstand the temperatures typically encountered in MBE machines without outgassing impurities.
One of the tools may include a solid source material reloader for easily melted materials. This tool may preferably be formed in the shape of a tube or cone with a small hole in the bottom. The tube is designed to receive solid source material. The handle of the tool is attached to a magnetically coupled transfer arm, similar to those employed in MBE machines to move substrate wafers into and out of the growth chamber. The handle may also be attached to a magnetically coupled transfer arm designed particularly for this purpose, which replaces the standard magnetically coupled arm used for substrate wafer loading. By manipulating the magnetically coupled arm, the tool can be moved such that the tube enters the desired effusion cell. The heater located within the effusion cell may then be used to heat the tube such that the surface material within the tube melts, and runs through the small hole in the base of the tube into the crucible located within the effusion cell. If necessary, an additional heater can be added to the tool to provide more heat to melt the source material.
For reloading solid source materials that are not easily melted, an xe2x80x9con-axisxe2x80x9d embodiment of the invention features an L-shaped support attached to either a handle or the magnetically coupled arm. The support has a compression clamp at its end which holds the solid source material in place. A wire is connected at one end to one side of the compression clamp, and at its other end to the magnetically coupled arm. Once the source material is in position relative to the cell for placement, the magnetically coupled arm may be turned about its axis, such that the wire pulls one side of the compression clamp, thereby opening the clamp and releasing the source material into the cell.
In another embodiment of the invention used for reloading solid source materials that are not easily melted, an xe2x80x9coff-axisxe2x80x9d design features an L-shaped support that is rotateably mounted to either an attachment or the magnetically coupled arm. A wire extends from the L-shaped support to the magnetically coupled arm, such that turning the arm about its axis causes the L-shaped support to raise or lower. The compression clamp on this embodiment features two wings extending from the back part of the clamp, whereby the wings are long enough such that their wingspan exceeds the diameter of the opening to the cell of interest. To load the source material into a cell, the clamp is directed into the cell until the wings strike the opening of the cell, thereby bending each side of the clamp until the source material is released into the cell.
In a still further embodiment of the invention that is a hybrid of those two embodiments described above, both the L-shaped support and a side of the compression clamp are connected to control wires. The control wire to the compression clamp extends back to the magnetically coupled arm and can be controlled as described above to release the source material into a cell. The wire extending to the L-shaped support may be connected to other control means within the MBE machine, such as a cart used to transport materials within the buffer chamber of the machine. Thus by moving the cart forward and backward, the operator may raise and lower the L-shaped support so that it may be aligned with a cell for source material reloading.
Finally, the source material reloading processes described above may incorporate either a one-port or two-port operation technique. The single-port method, using a buffer chamber in connection with the growth chamber, has already been described. In the two-port method, a buffer chamber or load-lock device is attached to the growth chamber. In the case where source material is being reloaded, that material is moved into the load lock, the load lock is brought to a vacuum, and then the load lock is opened such that a passageway exists between the growth chamber and load lock. The reload or other tool is located in a port roughly opposite the load lock. The tool may be attached to a magnetically coupled arm, or to a wobble stick that is attached to the port. Using the magnetically coupled arm or wobble stick, the tool is pushed into the load lock to retrieve the source material, and is then extended over to the cell for loading as explained above. Generally, the geometry of such a machine would have the port for the magnetically coupled arm or wobble stick on one side of the growth chamber, and the load lock and cell to be loaded on the other side of the growth chamber.
It is therefore an object of the invention to provide a tool and method for reloading and maintaining an apparatus without affecting the environment maintained within the apparatus, for example, having an ultra-high vacuum environment without venting the environment.
It is a further object of the invention to perform source material reloading in an MBE machine without translation of the cells being loaded.
It is a further object of the invention to provide a tool and method to reduce the down-time resulting from the reloading and maintenance of an MBE machine.